1. Field of the Invention
The present invention relates to infra-red detectors and more particularly but not exclusively to infra-red detectors for detecting infra-red radiation having a wavelength in the range 8-14 xcexcm.
2. Discussion of Prior Art
A variety of infra-red detectors have been described previously. For example, United Kingdom patent No. 1 488 258 describes a thermal radiation imaging device comprising a strip of photoconductive material in which infra-red radiation from a scene is scanned onto the photoconductive strip. The photoconductive strip described therein is a strip of cadmium mercury telluride, indium antimonide or lead tin telluride. U.S. Pat. No. 5, 016, 073 describes an integrated photoconductive detector comprising a heterostructure of cadmium mercury telluride alloys.
The above referenced detectors suffer from the disadvantage that they are not readily compatible with silicon integrated circuit fabrication techniques. It would be desirable to fabricate an infra-red detector which could be easily combined with a silicon integrated circuit in order to reduce the fabrication cost and possibly also to increase the signal to noise ratio.
It is an object of the present invention to provide an alternative infra-red detector.
The present invention provides an infra-red detector comprising a detector region and a collector region with a barrier region therebetween such that in operation incident infra-red radiation of a wavelength within an operating band is capable of exciting electrons within the detector region to an excited state in which the electrons have an energy corresponding to a sub-band energy level such that said electrons are able to cross the barrier region so as to be detectable in the collector region, characterized in that said detector region, barrier region and collector region are laterally displaced with respect to one another over a surface region of a semiconductor substrate.
An infra red detector in which the active regions are laterally displaced with respect to one another over a semiconductor substrate offers potential benefits of simplifying the manufacturing process compared with previous semiconductor infra-red detectors in which the active elements are disposed successively on a substrate.
The detector of the invention may be disposed on a silicon substrate.
A voltage tunable infra-red detector was proposed by Wheeler and Goldberg in IEEE Trans. Electron Devices, ED-22 (11), 1975, page 1001 which utilised inter-sub-band absorption in a silicon MOSFET inversion layer as the detection mechanism. Inter-sub-band transitions are described by Heitmann and Mackens in Physical Review B, Volume 33 Number 12, 1986, pages 8269 to 8283. In a metal-silicon dioxide-silicon device, a potential well may be formed at the interface of the silicon and the silicon dioxide, as shown in FIG. 1. In this narrow xe2x80x9ctriangularxe2x80x9d well, electron motion in the z-direction is quantised, with electron motion in the x-and y-directions being unaffected. The one particle energy spectrum of the electrons,             E      i        ⁢          (                        k          x                ,                  k          y                    )        =            E      i        +                            ℏ          2                ⁢                  k          x          2                            2        ⁢                  m          x                      +                            ℏ          2                ⁢                  k          y          2                            2        ⁢                  m          y                    
where mx and my are the effective masses in the x-and y-directions respectively and kx and ky are the wave numbers in the x-and y-directions, consists of a set of sub-bands (index i) that arises from the quantised motion perpendicular to the interface and continuous dispersion parallel to the interface.
Electrons may be transferred to a higher sub-band by absorption of an infra-red photon at an energy equal to the inter-sub-band energy separation. This process has a relatively high probability compared with free electron absorption because in this case k can be conserved. The sub-band energy levels can be calculated, assuming a triangular potential well, using the Wentzel-Kramers-Brillouin approximation. In this simple case, the calculation gives:             E      i        =                            (                                    ℏ              2                                      2              ⁢                              m                z                                              )                          1          /          2                    ⁢                        (                                                    3                ⁢                π                            2                        ⁢                          qF              s                                )                          2          /          3                    ⁢                        (                      i            +                          3              4                                )                          2          /          3                      ,
where q is the charge, Fs is the electric field and mz is the effective mass perpendicular to the interface.
The operation of an inter-sub-band infra-red detector depends on having an energy separation which is approximately equal to the photon energy to be detected. The energy separation increases as the doping level of the silicon increases. For the E1-E0 sub-bands, it is estimated that for a surface potential equal to the silicon band gap the energy separation increases from approximately 10 meV at a dopant level of 1015 cmxe2x88x923 to 100 meV at 1018 cmxe2x88x923 for {100} silicon. The energy separations for {110} and {111} surfaces are slightly greater for the same dopant level. The separation increases as the surface potential increases, with the maximum energy separation of the 0-1 transition being limited by the breakdown field of the oxide.
The above cited paper by Heitmann and Mackens describes experimental measurements on grating devices where the evidence for inter sub-band transitions was obtained by comparing the radiation transmitted through the device when the gate was biased above the threshold voltage to that when the gate was at or below the threshold voltage. The arrangement described in this paper thus needed a separate infra-red detector for detecting the transmitted radiation. The device described is not an infra-red detector but is a device for measuring inter-sub-band transitions.
The operation of the device of the invention may be controlled by gates adjacent the regions. Potentials applied to respective gates will control the passage of electrons between the differing regions in response to received radiation.
Preferably the silicon substrate has a {111} crystallographic orientation thereby avoiding the requirement for prisms or diffraction gratings on the surface of the detector to provide a component of the electric field of the incident radiation perpendicular to the surface. If the laterally disposed detector has an interdigitated structure, the efficiency of the device may be improved.